This invention relates generally to the field of mechanical connections and more particularly to flexure joints.
Effective implementation of precision control is largely influenced by the open-loop behavior of a machine. In particular, the presence of hard nonlinearities such as backlash and Coulomb friction results in significant deterioration of machine control. The elimination of hard nonlinearities enables effective and accurate position and force control of a precision machine.
Some classes of machines are particularly sensitive to the presence of backlash and Coulomb friction. Due to the physics of scaling, devices that operate on a microscopic scale are influenced by highly nonlinear surface forces to a much greater degree than those of a conventional scale. Consequently, a scaled-down micromachine is significantly more sensitive to Coulomb friction than its conventional-scale counterpart. Successful development of small-scale and micro-scale precision machines requires elimination or intelligent minimization of surface force behavior.
Machines that operate in a zero-gravity environment, such as satellites and spacecraft, are also particularly sensitive to Coulomb-type bearing friction, primarily because gravity is no longer the dominate mechanical influence.
Most conventional mechanisms rely on sliding and rolling at a fundamental level. Kinematic linkages, for example, cannot be constructed without revolute joints, which almost universally incorporate roller or journal bearings. In conventional machines, such designs can provide low friction rotation while bearing significant loads. Precision motion, however, as well as small-scale and zero gravity applications, require open-loop behavior that is devoid of any significant stick-slip behavior. A flexure-base joint, which utilizes deformation as a means of providing movement, is a viable alternative to the conventional revolute joint that does not exhibit any significant backlash or Coulomb friction and is free of lubricants. A diagram of a conventional flexure 2 is shown in FIG. 1. The basic characteristics of conventional flexure joints have been studied by several researchers [1,2,3,4].
[1] Goldfarb, M. and Celanovic, N., xe2x80x9cMinimum Surface-Effect Microgripper Design for Force-Reflective Telemanipulation of a Microscopic Environment,xe2x80x9d Proceedings of the ASME International Mechanical Engineering Conference and Exposition, November 1996.
[2] Horie M., Nozaki T., Ikegami K., and Kobayashi, F., xe2x80x9cDesign System of Super Elastic Hinges and its Application to Manipulator for Micro-Bonding by Adhesives,xe2x80x9d Proceedings of the International Symposium on Microsystems, Intelligent Materials, and Robots, pp. 185-188, 1995.
[3] Paros, J. and Weisbord, L., xe2x80x9cHow to Design Flexure Hinges,xe2x80x9d Machine Design, Vol. 37, No. 27, pp. 151-156, 1965.
[4] Ragulskis K., Arutunian M., Kochikian A., and Pogosian M., xe2x80x9cA Study of Fillet Type Flexure Hinges and Their Optimal Design,xe2x80x9d Vibration Engineering, pp. 447-452, 1989.
If properly designed, a flexure-based structure can approximate the motion of a complex kinematic linkage with negligible stick-slip friction and no backlash. Additionally, the absence of rolling and sliding surfaces produces a device that is free of lubricants and thus extremely conducive to clean environments. Conventional flexures, however have several significant deficiencies. One particularly restrictive deficiency is the limited range of motion. Depending on the flexure geometry and material properties, a flexure will begin exhibiting plastic deformation at ranges on the order of five to ten degrees of rotation. In contrast, an ideal revolute joint has an infinite range of motion.
Another significant problem with conventional flexures is the poor properties exhibited when subjected to multi-axis loading. An ideal revolute joint is infinitely rigid in all directions of loading except the desired axis of rotation. In contrast, a conventional flexure exhibits a significant stiffness along the desired axis of rotation and significant compliance along all other axes of loading. A flexure-based joint, for example, will exhibit twist-bend buckling when subjected to twisting. The multi-axis behavior of conventional flexure joints results in both kinematic and dynamic problems, especially when utilizing non-collocated control that relies upon kinematic transformations for task-space accuracy.
What is needed, then, is a joint that enables the implementation of precision spatially-loaded revolute joint-based machines with well-behaved kinematic characteristics and without the backlash and stick-slip behavior that would otherwise prevent precision control.
The new joint or flexure should: exhibit no backlash or stick-slip behavior; exhibit off-axis stiffnesses significantly greater than a comparable conventional flexure; enable greater range of motion than a comparable conventional flexure; and withstand more load than a conventional flexure. Such a joint is lacking in the prior art.
As mentioned previously, an ideal revolute joint is characterized by zero stiffness along the axis of rotation and infinite stiffness along all other axes of loading. Conventional flexure joints offer the benefit of zero backlash and Coulomb friction, but not without limitation. Conventional flexure joints are constrained to a small range of motion and are subject to significant stiffness along the axis of rotation and significant compliance along other axes. This application describes a new flexure that exhibits a considerably larger range of motion and significantly better multi-axis revolute joint characteristics than a conventional flexure.
The design of the joint is based upon contrasting the torsional compliance of an open section with its stiffnesses in compression and bending. The torsional mechanics of closed section and open section members are fundamentally and significantly different, while the bending and compressive mechanics of the members are quite similar. This difference in mechanics enables minimization of torsional stiffness and maximization of all other stiffnesses in a nearly decoupled manner.
One embodiment of the invention includes a split-tube flexure. The split-tube flexure includes a thin-waled shaft, a first arm, and a second arm. The shaft is not required to be thin-walled. It may be a generic tube having nearly any conventional of cross-section, including polygonal. Preferably, the tube is a thin-walled nearly closed split cylinder, or hollow shaft. The wall thickness is variable as well. Thus, this invention is not limited to thin-walled hollow shafts. Design requirements, such as a required strength or stiffness, will drive the design parameters (the structural dimensions and material properties).
The thin-walled shaft includes a wall, first and second ends, and a length between the first and second ends. The wall defines a lengthwise slit therein. The first link is attached to the shaft. The second link includes a length and is attached to the shaft transversely to the slit. In one embodiment, the shaft includes mounting holes at each end-to attach the first and second links. The mounting holes should be on a longitudinal axis opposite the slit.
Another embodiment of the split-tube flexure includes a second thin-walled shaft attached to the single split-tube flexure to form a compound split-tube flexure. Typically the shafts are aligned co-linear (end-to-end), though the only requirement is that the mounting holes of both tubes remain co-linear. The entire cross section of the first end of the first tube need not be co-linear with the entire cross section of the first end of the second tube, i.e. the respective centroids need not be aligned. A first end of the first tube typically faces a first end of the second tube. The first link is attached to the first ends of the tubes. A second link is attached to second ends of the tube. The links need not be attached at end points of course.
The invention includes a method of providing pivotal motion with a flexure joint. The method typically entails providing a first thin-walled shaft (or similar design constrained structure) having a lengthwise slit; attaching a first link to the thin-walled shaft, where the first link extends transversely to the slit (typically perpendicular). And attaching a second link to the first thin-walled shaft, the second link also extending transverse to the slit. Applying a force at the first arm, where the force has a vector component perpendicular to the slit. Applying the axial force will create reactive forces at the second arm, and thereby achieve minimal force resistance along the axis of rotation and relatively large force resistance along other axes of the flexure joint.
One objective of the invention is to provide a new flexure-based revolute joint that offers significantly better properties than a conventional flexure. More specific objectives of the invention are to provide a joint which exhibits little to no backlash or stick-slip behavior, and which exhibits off-axis stiffnesses significantly greater than a comparable conventional flexure. A further objective of the invention is to provide a mechanism which enables greater range of motion compared to a conventional flexure.
Another objective of the invention is to provide a joint which withstands more load than a conventional flexure.
A broader objective of the invention is to provide a joint that enables the implementation of a precision spatially-loaded revolute joint-based machines with well-behaved kinematic characteristics and without the backlash and stick-slip behavior that would otherwise prevent precision control. Another object of the invention is to provide this flexure in a lubricant free environment suitable for xe2x80x98cleanxe2x80x99 environments and space-based applications.